For me its probably the original Legend of Zelda. There was no other game that sparked such a sense of wonder, adventure, and discovery until I played Dark Souls decades later.
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I think I would have been around four years old. I distinctly remember playing before I entered Kindergarten. I remember being at my grandparents for some family-get-together, and my uncle was living with them at the time had a TV and NES hooked up in a side room to avoid having to interact with the rest of the family. I watched him play and I recall watching him bomb a mummy and get a key.
My parents are gamers as well, so we picked up an NES pretty early on and I was able to convince my uncle to loan us Zelda after he had beaten it. Racing through it against my parents and then NG+ is still one of my fondest gaming memories.